EVIDENCE FOR AN EARLY CAMBRIAN PROXIMAL IMPACT EJECTA LAYER IN THE NORTH

Introduction and aim of study: Relatively shallow marine successions of Early Cambrian age are preserved at many places along the Caledonian Mountain chain in Sweden and Norway. In the Tornetrask area, northern Sweden, the local Lower Cambrian deposits, up to about 100 m thick, are assigned to the Tornetrask and Grammajukku Formations [1, 2]. The Tornetrask Fm rests unconformably on the basement and comprises a number of sandand siltstone dominated members signaling changes in sea level during deposition. In this part of the sequence an enigmatic occurrence of a mostly 2-4 m thick, crystalline-rich, polymict breccia, the Vakkejokk Breccia (VB), has long attracted the attention of geologists working in the area [1, 3, 4]. The breccia occupies a stratigraphic position between the ‘Lower Siltstone mbr’ (LSM) and the ‘Red and Green Siltstone mbr’ (RGSM) of the Tornetrask Fm. The lower boundary is erosive and locally the breccia even rests directly on the basement. The VB contains a mixture of clasts of Lower Cambrian sediments as well as the crystalline basement. The clast size varies from gravel to boulders and some of the larger basement blocks exceed 100 m in length although being less than 10 m thick, but even basements rafts > 200 m long have been reported [4]. The VB has been reported from a series of exposures on the northern side of Lake Tornetrask, where it is semi-continuously exposed between the Vakkejokk and Tjaurajokk rivers, i.e. for a stretch of about 7 km [1, 4]. It can be traced further eastwards in disjunct river sections for another c. 7 km [cf. 1]. Thin conglomerates in the same stratigraphic position occur also south of Lake Tornetrask, e.g., at Luopakte, possibly representing an equivalent to the VB [1, 4], but may just as well be normal sedimentary conglomerates, marking a sequence boundary [cf. 2]. The breccia has previously been interpreted as a tillite or being fault-related [see summary by 1, 2]. However, the latter authors found the previous explanations unsatisfactory due to the rather local distribution and in particular the occurrence of the very large basement boulders and suggested the breccia to be impact related. This re-interpretation sparked the new investigations reported here. Methods: The study area was visited by Ormo and Nielsen during a one week field campaign in the summer of 2012 in order to study field relations of the VB and to collect samples that potentially could provide evidence of shock diagnostic for impact. Thin sections of VB samples obtained during the 2012 field campaign were studied by Alwmark using a Leitz 5-axes universal stage [5] mounted on an optical microscope in search of planar deformation features (PDFs) in quartz grains. The crystallographic orientations of identified PDFs were determined according to techniques described in [6, 7]. Results and discussion: Shock metamorphic features, in the form of PDFs, have been found in six quartz grains from one of the breccia samples (Fig. 1). Of the six shocked grains, four display a single set of PDFs, all oriented parallel to the basal plane c (0001). Two grains contain two sets; one set oriented along (0001) and one parallel to crystallographic plane ω {10 3}.